The invention relates to a method for packing polycrystalline silicon.
Polycrystalline silicon (polysilicon) is predominantly deposited from halosilanes such as trichlorosilane by means of the Siemens process, and then comminuted with minimum contamination into polycrystalline silicon chunks.
For applications in the semiconductor and solar industries, chunk polysilicon with a minimum level of contamination is desirable. Therefore, the material should also be packed with a low contamination level before it is transported to the customer.
Typically, chunk polysilicon is packed in plastic bags.
Chunk polysilicon is a sharp-edged, non-free-flowing bulk material. Therefore, in the course of packing, it has to be ensured that the material does not penetrate the customary plastic bags in the course of filling, or in the worst case, even completely destroys them. In order to avoid this, the prior art proposes various measures. US 2010/154357 A1, for example, envisages an energy absorber within the plastic bag.
However, such penetration of the bag can occur not just during packing but also in the course of transport to the customer. Chunk polysilicon is sharp-edged, and so, in the case of unfavorable orientation of the chunks in the bag, relative movement of the chunks with respect to the bag film can result in their cutting through it, or pressure of the chunks on the bag film can result in their penetrating it.
Chunks protruding from the bag packing can become unacceptably contaminated directly by surrounding materials and inner chunks by inflow of ambient air.
In addition, when packed silicon chunks are transported, there is unwanted post-comminution.
This is undesirable especially because the fines fraction which forms has been shown to lead to poorer operating performance for the customers. The result of this is that the fines fraction has to be sieved off again before further processing by the customer, which is disadvantageous.
This problem applies equally to crushed and classified, and to cleaned and uncleaned silicon, irrespective of the size of the package (typically bags containing 5 or 10 kg of polysilicon).
US 2010/154357 A1 proposes sucking the air out of the bag during sealing until a vacuum of 10 to 700 mbar arises.
US 2012/198793 A1 discloses sucking the air out of the bag before welding until a flat bag with a low air content arises.
These measures are unsuitable for preventing penetration.
This gave rise to the objective of the invention.